bridge: add support for isolated option

bridge: add support for isolated option

This patch adds support for the new isolated port option which, if set,
would allow the isolated ports to communicate only with non-isolated
ports and the bridge device. The option can be set via the bridge or ip
link type bridge_slave commands, e.g.:
$ ip link set dev eth0 type bridge_slave isolated on
$ bridge link set dev eth0 isolated on

Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David Ahern <dsahern@gmail.com>